1. Field of the Invention
The present invention relates to a mixed ionic conductor and an electrochemical device, such as a fuel cell or a gas sensor, using the same.
2. Description of the Prior Art
The applicant has long been actively developing mixed conductors of protons and oxide ions (see for example Publication of Unexamined Japanese Patent Application (Tokkai) No. H5-28820 or H6-231611). These mixed ionic conductors are basically perovskite oxides containing barium and cerium wherein a portion of the cerium has been substituted by the substitute element M, so as to achieve a high ionic conductivity (chemical formula: BaCe1−pMpO3−α). Especially, when the substitution amount p of the substitution element M is 0.16 to 0.23, the mixed ionic conductor has a high conductivity, higher even than zirconia-based oxides (YSZ: yttrium-stabilized zirconia), which conventionally have been used as oxide ionic conductors. As the substitution element M, rare earth elements are suitable, in particular heavy rare earth elements, because of their atomic radius and charge balance.
New fuel cells, sensors and other electrochemical devices using such materials as a solid electrolyte have been developed. The sensor characteristics and the discharge characteristics of fuel cells using such materials have been shown to be superior to prior devices. Other patent applications related to these materials are Tokkai H5-234604, Tokkai H5-290860, Tokkai H6-223857, Tokkai H6-290802, Tokkai H7-65839, Tokkai H7-136455, Tokkai H8-29390, Tokkai H8-162121, and Tokkai H8-220060.
However, these materials show some problems with regard to their chemical stability. For example, barium tends to precipitate in CO2 gas. To solve these problems, the applicant has proposed a counter-strategy in Tokkai H9-295866. However, even this counter-strategy is not perfect, and for example at low temperatures of 85° C. and 85% humidity, precipitation can be observed in shelf tests and boiling tests in water. Moreover, under high water vapor pressures as during discharge of the fuel cells, barium can be seen to precipitate near the platinum electrodes. Furthermore, with gas sensors, there is the problem of maintaining high ion conductivity at lower temperatures over a long time and the problem of raising the acid resistance of the oxide itself.